Size-controlled syntheses and hydrophilic surface modification of Fe 3O4, Ag, and Fe3O4/Ag heterodimer nanocrystals

Article abstract of DOI:10.1039/c0dt00965b

High-quality, monodisperse, and size-controlled Fe₃O ₄, Ag, and bifunctional Fe₃O₄/Ag heterodimer nanocrystals (NCs) have been synthesized successfully. In the synthesis of Fe₃O₄ NCs, dodec

Full text of DOI:10.1039/c0dt00965b

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www.rsc.org/dalton | Dalton Transactions
Size-controlled syntheses and hydrophilic surface modification of Fe O , Ag,
3
4
and Fe O /Ag heterodimer nanocrystals†
3
4
a
a
a
a
a
b
a
Xiaomin Li, Honglei Si, Jin Zhong Niu, Huaibin Shen, Changhua Zhou, Hang Yuan, Hongzhe Wang,*
b
a
Lan Ma* and Lin Song Li*
Received 15th July 2010, Accepted 1st September 2010
DOI: 10.1039/c0dt00965b
High-quality, monodisperse, and size-controlled Fe
3
O
4
, Ag, and bifunctional Fe
3
O
4
/Ag heterodimer
nanocrystals (NCs) have been synthesized successfully. In the synthesis of Fe
3
O
4
NCs, dodecanol was
chosen as the substitute of 1,2-hexadecanediol and “size control” was achieved by simply adjusting the
proportion among the ligands instead of utilizing seed-mediated growth. In the synthesis of Ag NCs,
organometallic silver acetylacetonate (Agacac) was used as precursors and tunable particle size could
be easily obtained by adjusting the reaction temperatures. By using different sized Fe
Fe /Ag heterodimer NCs with particle sizes tuned from 5 to 16 nm for Fe and 4 to 8 nm for Ag
were successfully synthesized and superparamagnetism were maintained. We found that the size of Ag
attached on the Fe NCs relied on the size of Fe seed. UV-vis absorption spectra and TEM
investigations revealed that the bigger the Fe NCs seed used, the bigger the Ag NCs that were
3
4
O NCs as seeds,
3
O
4
3
O
4
3
O
4
3
O
4
3
O
4
obtained from the heterodimer NCs. In addition, we demonstrated that all of these NCs were
successfully transferred into water by surface modification with biocompatible carboxylic acid groups,
which made them meet the basic requirement for biolabeling and biomedical applications.
Introduction
Fe
3
O
4
NCs as an example: 4 nm nearly monodisperse Fe
3
O
4
NCs could be synthesized by using ferric acetylacetonate as the
precursor in the presence of 1,2-hexadecanediol, oleylamine, and
oleic acid in phenol ether. By the introduction of seed-mediated
growth method, larger monodisperse magnetite nanoparticles of
up to 20 nm in diameter could be synthesized. For the synthesis
of noble metal NCs, water-soluble noble salt precursors were
commonly used. For example, Ag NCs were synthesized by
thermal decomposition silver nitrate in octadecylamine (ODA)
Nano-sized materials have attracted a great deal of attention from
researchers in various areas for their fundamental size- and shape-
dependent novel magnetic, optical, and other unique properties.
1–3
Magnetic nanocrystals (NCs) such as iron oxide, manganese oxide,
and noble metal NCs such as Au and Ag are especially important
for their potential applications in fields of high-density magnetic
storage, magnetic resonance imaging (MRI), molecular adsorp-
4–17
tion, and catalysts, chemical sensing, biological applications.
26
has been reported by Li group. Based on the very successful
Recently, various heterostructured NCs composed of noble metal
and magnetic oxide have been synthesized, for the purpose of
merging the properties of different materials and creating the
syntheses of magnetic and noble metal NCs, synthesis of magnetic
noble metal heterodimer NCs was well developed as well. As far
as we know, the investigations on magnetic metal heterodimer
NCs are mainly focused on the morphology-controlled syntheses,
the particle size related effects between the magnetic and noble
metal NCs on the synthesis of heterodimer has rarely been
discussed.
18–22
possibility for new multifunctional applications.
For example,
live cells can be manipulated using an applied magnetic field
and imaging with two-photon fluorescence microscopy under
20
assistance of such multifunctional heterostructured NCs. In
addition, the metal-magnetic multifunctional heterostructured
NCs could also be used in multimodal biological detections: noble
metals for chip-based biosensing and magnetic resonance contrast
Here we report on the size-controlled syntheses of monodisperse
Fe
3
O
4
(4–20 nm), Ag (4–9 nm), and bifunctional Fe
3
O
4
/Ag
heterodimer NCs. In the synthesis of Fe
3
O
4
NCs, dodecanol
22
effects of superparamagnetism.
There are many methods for the synthesis of monodisperse
was chosen as the substitute of 1,2-hexadecanediol and “size-
control” was achieved by simply adjusting the ratio of different
ligands instead of seed-mediated growth. For the synthesis of Ag
NCs, organometallic silver acetylacetonate (Agacac) was chosen
as precursor and particle size could be tuned easily by adjusting
23–25
magnetic nanocrystals.
Among them, the most successful and
common strategy is the thermolysis of organometallic precursors
24
in complex organic solvent systems reported by Sun et al. Take
the reaction temperatures. By using differently sized Fe
3
4
O NCs as
a
seeds, Fe O /Ag heterodimer NCs with particle sizes tuned from
Key Laboratory for Special Functional Materials of Ministry of Education,
3
4
Henan University, Kaifeng, 475004, P. R. China. E-mail: whz@henu.edu.cn,
lsli@henu.edu.cn
Graduate School at Shenzhen, Tsinghua University, Shenzhen, 518055,
P. R. China. E-mail: malan@sz.tsinghua.edu.cn
5 to 16 nm for Fe O and 4 to 8 nm for Ag were successfully
3
4
synthesized and the superparamagnetism was maintained. In our
synthesis, we found that the size of Fe seed determined the
size of attached Ag NCs of the herterodimer. UV-vis absorption
spectra and TEM investigations revealed that the bigger the Fe
b
3
O
4
†
Electronic supplementary information (ESI) available: Fig. S1–S6. See
DOI: 10.1039/c0dt00965b
3
O
4
1
0984 | Dalton Trans., 2010, 39, 10984–10989
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View Article Online
NCs seed used, the bigger the Ag NCs obtained for the final
heterodimer NCs. Furthermore, size-distribution, magnetization
to room temperature, acetone (10 mL) was added to the reaction
mixture to precipitate the heterodimer NCs. A dark yellowish
brown solid was collected by centrifugation at 16 000 rpm. The
final product could be easily redispersed in hexanes or toluene
behaviors, and optical properties of the as-prepared Fe
3
4
O /Ag
heterodimer NCs were investigated in detail. In order to make
those NCs suitable for biomedical applications, surface modifi-
cation with biocompatible carboxylic acid group was carried out
for storage. The strategies of synthesis of 15 nm Fe
Ag and 5.5 nm Fe /4.5 nm Ag heterodimer NCs are the same
as the synthesis of 12 nm Fe /6 nm Ag heterodimer NCs with
being used as seeds.
3
4
O /7.5 nm
3
O
4
and Fe
3
O
4
, Ag, and Fe
3
O
4
/Ag were all transferred into water
3
O
4
successfully. This makes such NCs suitable for further conjunction
with biomolecules, such as antibodies, proteins, peptides, and
nucleic acids and can serve as new building blocks for the
next generation of multimodal biomedical applications including
bioseparation, sensing, and imaging.
different sizes of Fe
3
O
4
Characterization
Transmission electron microscopy (TEM) studies were performed
using a JEOL JEM-100CX II microscope with an accelerating
voltage of 100 kV. High resolution transmission electron mi-
croscopy (HRTEM) observations were performed with a JEOL
JEM-2010 microscope with an accelerating voltage of 200 kV,
accompanied by selected area electron diffraction (SAED) studies.
TEM samples were prepared by placing a drop of the nanocrystal
solution onto a carbon coated copper grid. Phase determina-
tion of the products was carried out by a Philips X¢Pert Pro
Experimental
Chemicals
Oleic acid (OA, 90%), oleylamine (OAM, 70%), 1-octadecene
(
ODE, 90%), and iron acetylacetonate (Fe(acac)
obtained from Aldrich. 2,4-Pentanedione (98%), triethylamine
99%), hexanes (analytical grade), methanol (analytical grade),
3
, 97%) were
(
˚
X-ray diffractometer using Cu-Ka radiation (l = 1.54 A). Room
ethanol (analytical grade), and acetone (analytical grade) were
purchased from Tianjin Chemical Reagents Co. Ltd. Silver nitrate
temperature UV-vis absorption and PL spectra were measured
with an Ocean Optics spectrophotometer (mode PC2000-ISA).
Fourier transform infrared (FTIR) spectra were recorded on a
Nicolet AVATAR 360 spectrometer. X-Ray photoelectron spec-
trum (XPS) measurements were performed on an AXIS ULTRA
X-ray photoelectron spectroscope, using monochromatised Al Ka
radiation with an anode voltage of 15 kV and emission current of
(
AgNO , 99.8%) were obtained from Beijing Chemical Reagents
3
Co. Ltd. All the chemicals were used as received without any
further purification.
Typical synthesis of Fe
3
4
O NCs
3
mA. The magnetic measurements were performed with a Lake
For the synthesis of 12 nm Fe
3
O
4
NCs, Fe(acac) (0.3532 g,
3
Shore 7410 at room temperature.
1
.0 mmol) was mixed with ODE (10 mL), OA (0.2825 g, 1.0 mmol),
OAM (0.8025 g, 3.0 mmol) and dodecanol (0.5590 g, 3.0 mmol),
◦
then heated to 270 C under stirring and nitrogen flow for an
Results and discussion
hour. Ultimately, a clear black solution containing Fe
3
O NCs
4
Synthesis of Fe
3
4
O NCs
was obtained and the resulting NCs were purified with acetone,
methanol and hexanes. Fe
3
O
4
NCs with different particle sizes of
: OA : OAM : dodecanol equal to
The synthesis of magnetic NCs with controlled sizes has long
been of scientific and technological interest because of their
broad applications. For all applications, synthesis techniques
which provide precise control over NCs grain sizes and size
distributions are crucial in that the particles have uniform physical
and chemical properties. However, the preparation of magnetic
NCs with the desired size and acceptable size distribution without
particle aggregation has consistently been aspired after. Recently,
we reported a chemical synthesis of manganese oxide NCs in
multi-ligands systems (the mixture of oleylamine, oleic acid,
4
nm, 15 nm, and 20 nm can also be synthesized, with different
reactants ratios of Fe(acac)
3
0
.5 : 2 : 3 : 3, 2 : 1 : 3 : 3, 2 : 0.5 : 3 : 3, respectively.
Typical synthesis of Ag NCs
For the synthesis of 9 nm Ag NCs, Agacac (0.0207 g, 0.1 mmol) was
mixed with 5 mL OAM and heated to 200 C under stirring and
nitrogen flow for half an hour. Finally, a clear dark brown solution
containing Ag NCs was obtained and the resulting NCs were
purified using the same procedure as Fe
◦
25
and dodecanol). That was the first time to use inexpensive
dodecanol as the substitute of expensive 1,2-hexadecanediol. Here
we successfully extended this approach to synthesize high quality
3
4
O NCs. The strategies
for the synthesis of 4 nm Ag NCs were the same but the reaction
temperature was set to 110 C.
◦
Fe
to 20 nm.
In the multi-ligands system for the synthesis of Fe
3
O
4
NCs and the size of Fe
3
O NCs can be well controlled from
4
4
Typical synthesis of Fe
3
4
O /Ag heterodimer NCs
3
O NCs,
4
For the synthesis of 12 nm Fe
acetone (10 mL) was added to 3.5 mL (~0.1 mmol) reaction solvent
of Fe NCs. A black solid was precipitated and collected by
3
O
4
-6 nm Ag heterodimer NCs,
it was found that all reaction conditions include reactant ratios
and temperatures have important influences on size and size
distribution of as-synthesized NCs (Fig. S1, ESI†) in the multi-
3
O
4
centrifugation and then redispersed in a mixture of 0.1 mmol
Agacac (0.0207 g) and 5 mL OAM. The reaction mixture was
heated to 110 C and maintained 5 h under stirring and nitrogen
ligands system we chose for the synthesis of Fe
amount of OA for example: when the molar ratio of Fe(acac)
: OA : OAM : dodecanol was set to 1 : 0.5 : 3 : 3, 1 : 1 : 3 : 3, and
1 : 2 : 3 : 3, monodisperse 16 nm, 12 nm, and 5 nm Fe NCs
3
O NCs. Take the
4
3
◦
flow. The initial black solution turned to yellowish brown, which
3
O
4
meant the formation of Fe
3
O
4
/Ag heterodimer NCs. Upon cooling
were synthesized, respectively (Fig. S1). That is to say, as the
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concentration of OA in ODE increased, the particle size of the
monodisperse NCs decreased systematically. This might originate
external magnetic field (Fig. S2B, ESI†). The selected area electron
diffraction (SAED) patterns showed six distinct rings and the
calculated d-spacings matched well with interplanar distances of
from excessive OA attach on the surface of the Fe
3
4
O NCs, which
restricted continuous growth of the NCs. As shown in Fig. S1, we
did a series of control experiments to investigate the relationships
face-centered cubic (fcc) Fe
nanocrystal was studied by high-resolution TEM (HRTEM). Fig.
1F shows the HRTEM image of an isolated 20 nm Fe NCs.
The clear lattice fringes indicate that the particle is a single
3
O
4
(Fig. 1E). The structure of a single
between size, size distribution of the Fe
ratios (or temperatures). It was found that the amounts of OA
and Fe(acac) played important roles for the control of NCs sizes.
3
O
4
NCs, and the reactant
3
O
4
27
˚
3
crystal. The distance between two adjacent planes is 2.53 A,
which corresponds to the (311) plane of the fcc structured Fe
The X-ray diffraction (XRD) study shows that the as-
synthesized Fe NCs have a fcc structure and the peak width
of the diffraction pattern is size dependent. Fig. 2 depicts the
XRD patterns of as-prepared Fe NCs with different sizes.
Furthermore, it was found that OAM, the same as OA, could also
influence the size of the as-synthesized NCs. As the concentration
of OAM in ODE increased, the size of the monodisperse NCs
decreased systematically. But there was a point quite different
from OA, when the concentration of OAM decreased to less than
3
4
O .
3
O
4
3
O
4
0
.3 mmol per 10 mL ODE, only NCs with broad size distribution,
The positions and relative intensity of all diffraction peaks match
well with (220), (311), (400), (422), (511), (440), and (533) planes
or lumpish particles aggregation were obtained. Dodecanol was
used as reducing agent, which may not have an important effect
on particle size and size distribution. So, an excess amount
of dodecanol was used in our synthesis. Reaction temperature,
another parameter in our synthesis, was set at 270 C. If the
temperature was set higher or lower than 270 C, the synthesized
of standard Fe
respectively. The sizes of Fe
3
O
4
powder diffraction data (JCPDS, 85–1436),
NCs estimated from Scherrer’s
3
O
4
formula are consistent with that determined by statistical analysis
based on TEM images.
◦
◦
NCs with broad size distribution was obtained. To sum up, the
amounts of OA and Fe(acac)
the control of NCs sizes. So, the size-control of Fe
3
played most important roles in
NCs was
3
O
4
achieved mainly by adjusting the amounts of OA and Fe(acac)
here.
3
Conducted by the method mentioned above, monodisperse
Fe NCs with sizes in 4–20 nm range were synthesized success-
3
O
4
fully. Typical transmission electron microscopy (TEM) images of
the as-synthesized NCs with average diameters of 20 nm, 15 nm,
1
2 nm, and 4 nm are presented in Fig. 1A–D, respectively. As can
Fig. 2 X-Ray diffraction patterns of Fe
3 4
O NCs with various sizes: 20 nm,
be seen from the TEM images, the NCs generally self-assembled
into a monolayer on the carbon substrate of the TEM grid without
1
5 nm, 12 nm, and 4 nm from top to bottom, respectively.
the formation of aggregates. The as-prepared Fe
be easily dispersed in various organic solvents, such as hexanes,
chloroform, or toluene, and shows ferrofluidic behavior under an
3
O NCs could
4
In order to make as-prepared Fe
applications, surface modification is required for the transfer of
hydrophobic products to water. Following the surface modification
3
O NCs suitable for biomedical
4
28
method reported by our group recently, the as-prepared Fe
3
O
4
NCs were further transferred into water successfully (Fig. S2,
ESI†). The obtained water-soluble NCs was modified by biocom-
patible carboxylic acid groups which were ideal for binding to
biomolecules, such as antibodies, proteins, peptides, and nucleic
acids.
Synthesis of Ag NCs
The synthesis of Ag NCs was conducted by one-pot method, and
Agacac was introduced to synthesize Ag NCs. OAM was chosen
for the preparation of Ag NCs because it can be used not only as
a surface-capping agent, but also as a reducing agent. In a typical
synthesis, a mixture of 0.1 mmol Agacac and 5 mL OAM was
◦
add into 25 mL beaker and then heated to 200 C with stirring
under a nitrogen flow. The color of the solution changed from
colorless to light yellow and finally became dark brown, indicating
the formation of Ag NCs. By this approach, Ag NCs with tunable
sizes were easily synthesized by adjusting the reaction temperature.
Fig. 3A and B show typical TEM images of Ag NCs with average
◦
diameters of 4 and 9 nm, which were prepared at 110 C and
2
Fig. 1 TEM images of as-synthesized Fe
C) 12 nm, and (D) 4 nm. (E) Selected area electron diffraction (SAED)
pattern acquired from (A). (F) High-resolution TEM image of a single
0 nm Fe NCs.
3 4
O NCs: (A) 20 nm, (B) 15 nm,
◦
00 C, respectively. It can be seen that the Ag particles have a
narrow size distribution and the sizes of particles are influenced
by the reaction temperature forcefully. Different from the synthesis
(
2
3 4
O
1
0986 | Dalton Trans., 2010, 39, 10984–10989
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View Article Online
prepared Fe
3
O
4
/Ag heterodimer NCs confirmed that our synthetic
strategy is effective. By using differently sized Fe
Fe /Ag heterodimer NCs with particle sizes tunable from 5 to
6 nm for Fe and 4 to 8 nm for Ag were successfully synthesized.
Fig. 4A and B shows TEM images of Fe /Ag heterodimer NCs
with Fe at around 16 nm and 5 nm, respectively, and Ag at
around 8 nm and 4 nm, respectively. In the TEM images, the Ag
particles often appear black and Fe are light colored, which is
3
O
4
NCs as seeds,
3
O
4
1
3
O
4
3
O
4
3
O
4
3
O
4
partly because Ag has a higher electron density and allows fewer
electrons to transmit. The SAED pattern (Fig. 4C) shows a few
distinct diffraction rings and their calculated d-spacings match
well with the major d-spacings of fcc structured Fe
3
O
4
and fcc
structured Ag. Fig. 4D is a typical HRTEM image of a heterodimer
Fig. 3 TEM images of Ag NCs synthesized using Agacac as precursor
at 110 C (A) and 200 C (B). (C) SAED pattern acquired from (B). (D)
High-resolution TEM image of a 9 nm Ag NCs.
NCs with Fe
adjacent planes in Fe
3
O
4
at 5 nm and Ag at 4 nm. The distance between two
˚
O is measured to be 2.53 A, corresponding
◦
◦
3
4
˚
to (111) planes in the fcc structured Fe
3
O and that in Ag is 2.04 A,
4
resulting from a group of (200) planes in fcc structured Ag. More
HRTEM images for different sized heterodimer NCs are shown in
Fig. S5, ESI†.
of Fe
3
O NCs, the ratio among the reactants has an inconspicuous
4
influence on the size of Ag NCs. As can be seen in Fig. S3,† B and
D are TEM images of aliquots taken at 30 min and 5 h when
the reaction temperature was set at 200 C. It was found that by
◦
extending the reaction time the sizes of the particles were nearly
the same and the size distribution did not show any change. So
the size-control of Ag NCs discussed here was achieved just by
adjusting the reaction temperatures.
The phase identification of as-synthesized Ag NCs was obtained
from electron diffraction and X-ray diffraction. Fig. 3C is a SAED
pattern acquired from 9 nm NCs assembly. Table 1 displays the
measured lattice spacing based on the rings in the diffraction
pattern and compares them to the known lattice spacing for bulk
Ag along with their respective (hkl) indexes (JCPDS, 04–0783).
The XRD patterns of as-synthesized Ag NCs are shown in Fig.
◦
◦
◦
S4.† From the XRD pattern, the peaks at 38.2 , 44.3 , 64.4 , and
7
◦
7.5 correspond to the (111), (200), (220), and (311) reflections of
3 4 3 4
Fig. 4 TEM images of 16 nm Fe O /8 nm Ag (A) and 5 nm Fe O /4 nm
fcc structured Ag, which is in accordance with the result of SAED
pattern. The inset shows the photographs of as-prepared Ag NCs
before and after transfer into water by surface modification (Fig.
S4). The HRTEM images reveal the highly crystalline nature with
Ag (B) heterodimer NCs. (C) SAED pattern acquired from A (the rings
signed with black and red words correspond to Fe O and Ag, respectively).
3
4
3 4
(D) High-resolution TEM image of a single 5 nm Fe O /4 nm Ag
heterodimer NCs.
˚
an interplanar spacing of 2.04 A, which corresponding to (200)
The crystalline nature of the heterodimer NCs can also be
characterizedbyXRDinvestigations. Fig. 5A shows representative
planes of fcc structured Ag NCs (Fig. 3D).
XRD patterns of the heterodimer NCs with Fe
5 nm) and Ag at 8 nm (4 nm). The positions and relative intensity
of all diffraction peaks match well with standard Fe and Ag
powder diffraction data, indicating that the synthesis yields fcc
structured Fe and fcc structured Ag. It can be seen that the
3
O at around 16 nm
4
Synthesis of Fe
3
O
4
/Ag heterodimer NCs
(
On the base of the synthesis of size-controlled Fe
NCs, we tried our best to extend the work to synthesize Fe
heterodimer NCs and achieved a great success.
In a typical synthesis, the Fe
first synthesized as described above. Heterodimer NCs were
then formed by reducing Agacac in OAM under the presence
3
O
4
and Ag
/Ag
3
O
4
3
O
4
3
O
4
3
4
O NCs of a desired size were
peaks of minor particle-sized heterodimers (red curve) are broader
than the peaks of bigger heterodimers (green curve). To obtain
more information on the components of heterodimer NCs, X-ray
photoelectron spectroscopy (XPS) was introduced. As shown in
Fig. 6, we examined the Fe 2p, O 1s, and Ag 3d of the sample,
respectively. The Fe 2p core levels are split into 2p1/2 and 2p3/2
components, which is due to the spin–orbit coupling (Fig. 6B).
It can be seen that the shake-up satellite signal at 719.0 eV is
very weak (almost disappeared), which further proves that the
of pre-synthesized Fe
3
4
O seed NCs. TEM investigations of as-
Table 1 Measured lattice spacing, d ( A˚ ), based on the rings in Fig. 3C
and standard atomic spacing for Ag along with their respective hkl indexes
from the PCPDFWIN database.
Ring
29
product combined with Ag NCs is Fe
3
O
4
rather than g-Fe
2
3
O .
1
2
3
4
5
6
The O 1s spectrum is shown in Fig. 6C. The observed peak is
asymmetric with a shoulder on the higher binding energy side,
indicating more phases of O element. There maybe three phases
d/A˚
2.35
2.36
111
2.05
2.04
200
1.47
1.45
220
1.25
1.23
311
0.92
0.94/0.91
331/420
0.82
0.83
422
Ag/A˚
hkl
of O element contribute to the O 1s spectrum: O element in Fe
3
O
4
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The combination of Fe
3
O
4
and Ag can also lead to the change
of optical properties of the Ag NCs. In order to investigate the
influence of Fe NCs on the heterodimer NCs, a series of
experiments were performed with and without the participation
of Fe . Fig. 7B shows the UV-vis spectra of the results. Once Ag
NC attached to Fe , it showed a broader absorption feature at
14 nm (green line), a ~5 nm red-shift from the absorption feature
compare to pure Ag NCs which was prepared at the same reaction
conditions without Fe . Comparing the absorption change of
3
O
4
3
O
4
3
O
4
4
3
O
4
different sized of Ag NCs (Fig. 7A), it was found that the maxima
absorption center was proportional to the size of Ag NCs which
30
was consistent with the literature. So we conclude that the “red-
shift” after the combination with Fe may result from the bigger
size of Ag NCs that were obtained when integrated with Fe
To further make it clear how Fe NCs influence the growth of
Ag NCs, the average particle diameters of the heterodimer NCs
were estimated by statistical analysis of the TEM images and the
results were shown in Fig. 8. It was found that when integrated
3
O
4
3
4
O .
3
O
4
Fig. 5 (A) X-Ray diffraction patterns of 16 nm Fe
nm Fe /4 nm Ag heterodimer NCs. The bottom lines represent the
XRD patterns of fcc structured of bulk Fe (black) and Ag (blue).
B) Hysteresis loops of 5 nm Fe , 5 nm Fe /4 nm Ag, and 16 nm
Fe /8 nm Ag heterodimer NCs at room temperature.
3 4
O /8 nm Ag and
5
3 4
O
3
O
4
O
3 4
(
3
O
4
3
O
4
with Fe
than the Ag NCs from synthesis without Fe
consistent with inference obtained from UV-vis spectra shown in
Fig. 7. With the sizes of Fe change from 16 nm to 12 nm and
nm, the sizes of combined Ag NCs change from 8 nm to 6 nm
and 4 nm, respectively, as shown in Fig. 8A–E. That is to say, the
bigger Fe NCs were used the heterodimer NCs with the bigger
3
O
4
, the average particle diameter of Ag NCs was greater
3
O
4
. The result is
3
O
4
5
3
O
4
Ag NCs were obtained.
Fig. 7 UV-vis absorption spectra of Ag NCs with different sizes (A) and
3 4
Fig. 6 XPS survey, Fe 2p, O 1s, and Ag 3d spectra of 5 nm Fe O /4 nm
Ag heterodimer NCs.
1
3 4
6 nm Fe O /8 nm Ag heterodimer NCs (B).
NCs, O element in organic ligand, and contamination oxygen from
air. The binding energies of Ag 3d3/2 and Ag 3d5/2 for Ag NCs
were 373 eV and 367 eV with a peak splitting of 6 eV, which was
consistent with the standard reference XPS spectrum of Ag.
The interaction between the nanoscale Ag and Fe
a change of magnetization behavior of the Fe NCs. Fig. 5B
shows the hysteresis loops of 5 nm Fe NCs, 5 nm Fe /4 nm
Ag, and 16 nm Fe /8 nm Ag heterodimer NCs measured at
room temperature. It can be seen that 5 nm Fe NCs were
superparamagnetic and magnetization (M) value was ~24 emu
3
O
4
leads to
3
O
4
3
O
4
3
O
4
3
O
4
3
O
4
-
1
g
at an applied field (H) of 18 kOe, which was lower than the
-
1
saturation magnetization value of bulk magnetite (~90 emu g ).
Like 5 nm Fe NCs, the 5 nm Fe /4 nm Ag heterodimer NCs
were superparamagnetic too. But the magnetization value of 5 nm
Fe
much smaller than that of 5 nm Fe
compared the magnetization values of different size of Fe
heterodimer NCs and found the magnetization value increased
3
O
4
3
O
4
Fig. 8 TEM images and particle size distribution analysis of the typical
samples of 16 nm Fe /8 nm Ag (A, B), 12 nm Fe /6 nm Ag (C, D),
and 5 nm Fe O /4 nm Ag (E, F) heterodimer NCs.
-
1
3
O
4
/4 nm Ag heterodimer NCs was ~10 emu g which was
NCs alone. In addition, we
/Ag
O
3 4
O
3
4
3
O
4
3
4
3
O
4
The heterodimer NCs were capped with a hydrophobic layer,
such that they were soluble in organic nonpolar solvents (such
as hexanes, toluene and chloroform). In order to make these
with the size increase of Fe
applied field (H) (Fig. 5B).
3
4
O /Ag heterodimer NCs at same
1
0988 | Dalton Trans., 2010, 39, 10984–10989
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NCs useful for biological applications, phase transfer experiments
were conducted, and the NCs were finally rendered water-soluble.
The phase transfer experiments and results are shown in Fig.
S6, ESI†. The obtained water-solubility NCs were modified
by biocompatible carboxylic acid groups which were ideal for
conjugating with biomolecules, such as antibodies, proteins,
peptides, and nucleic acids. With such high water-solubility and
biocompatibility, the as-prepared NCs potentially can serve as a
new nanoplatform technology for the next generation multimodal
biomedical applications including bioseparation, sensing, and
imaging.
References
1
2
L. Brus, J. Phys. Chem., 1986, 90, 2555–2560.
C. B. Murray, C. R. Kagan and M. G. Bawendi, Annu. Rev. Mater. Sci.,
2000, 30, 545–610.
3 A. P. Alivisatos, Science, 1996, 271, 933–937.
4
5
Y. Chabre and J. Pannetier, Prog. Solid State Chem., 1995, 23, 1–130.
M. S. Whittingham and P. Y. Zavalig, Solid State Ionics, 2000, 131,
1
09–115.
6 Y. F. Shen, P. R. Zerger, N. R. DeGuzman, L. S. Suib, L. I. McCurdy,
D. Potter and C. L. O’Young, Science, 1993, 260, 511–515.
7
8
M. Yin and S. O’Brien, J. Am. Chem. Soc., 2003, 125, 10180–10181.
O. Giraldo, S. L. Brock, W. S. Willis, M. Marquez and S. L. Suib, J.
Am. Chem. Soc., 2000, 122, 9330–9331.
9
A. R. Armstrong and P. G. Bruce, Nature, 1996, 381, 499–500.
1
1
0 A. H. De Vries, L. Hozoi and R. Broer, Phys. Rev. B: Condens. Matter
Conclusions
Mater. Phys., 2002, 66, 35108.
1 Y. M. Huh, Y. W. Jun, H. T. Song, S. Kim, J. S. Choi, J. H. Lee, S. Yoon,
K. S. Kim, J. S. Shin, J. S. Suh and J. Cheon, J. Am. Chem. Soc., 2005,
127, 12387–12391.
In summary, monodisperse size-tuneable Fe
tional Fe /Ag heterodimer NCs have been synthesized with a
facile solution route. In the synthesis of Fe NCs, dodecanol
3
O
4
, Ag, and bifunc-
3
O
4
1
1
1
2 M. Haruta, CATTECH, 2002, 6, 102–115.
3
O
4
3 C. J. Zhong and M. M. Maye, Adv. Mater., 2001, 13, 1507–1511.
4 S. D. Evans, S. R. Johnson, Y. L. L. Cheng and T. H. Shen, J. Mater.
Chem., 2000, 10, 183–188.
was chosen as the substitute of 1,2-hexadecanediol and “size-
control” was achieved by simply adjusting the proportion among
the multi-ligands without any seed-mediated growth method. In
the synthesis of Ag NCs, organometallic silver acetylacetonate
1
5 M. Zayats, A. B. Kharitonov, S. P. Pogorelova, O. Lioubashevski, E.
Katz and I. Willner, J. Am. Chem. Soc., 2003, 125, 16006–16014.
6 W. Baschong and N. G. Wrigley, J. Electron Microsc. Tech., 1990, 14,
1
(
Agacac) was chosen as precursor and tunable sizes were easily
obtained by adjusting the reaction temperatures. Under the
conductions of the syntheses of Fe and Ag NCs, Fe /Ag
bifunctional heterodimer NCs with particles sizes tuned from
to 16 nm for Fe and 4 to 8 nm for Ag were successfully
synthesized. The current synthetic procedure is very simple, low-
cost, and highly reproducible. As-prepared hydrophobic Fe , Ag
and bifunctional Fe /Ag heterodimer NCs were successfully
3
13–323.
17 W. Jahn, J. Struct. Biol., 1999, 127, 106–112.
1
8 (a) W. Shi, H. Zeng, Y. Sahoo, T. Y. Ohulchanskyy, Y. Ding, Z. L. Wang,
M. Swihart and P. N. Prasad, Nano Lett., 2006, 6, 875–881; (b) H. Ding,
C. M. Shen, C. Chao, Z. C. Xu, C. Li, Y. Tian, X. Z. Shi and H. J. Gao,
Chin. Phys. B, 2010, 19, 066102.
3
O
4
3
O
4
5
3
O
4
19 (a) H. Yu, M. Chen, P. M. Rice, S. X. Wang, R. L. White and S. Sun,
Nano Lett., 2005, 5, 379–382; (b) G. Lopes, J. M. Vargas, S. K. Sharma,
F. Beron, K. R. Pirota, M. Knobel, C. Rettori and R. D. Zysler, J. Phys.
Chem. C, 2010, 114, 10148–10152.
3
O
4
3
O
4
transferred into water and modified by carboxylic acid groups
which are ideal for conjugating with biomolecules. With such
high water-solubility and biocompatibility, the as-prepared NCs
provide the potential to serve in many multifunctional biomed-
ical applications, such as multimodal imaging and detection
probes.
20 J. Jiang, H. Gu, H. Shao, E. Devlin, G. C. Papaefthymiou and J. Y.
Ying, Adv. Mater., 2008, 20, 4403–4407.
1 S. H. Choi, H. B. Na, Y. Il Park, K. An, S. G. Kwon, Y. Jang, M. Park,
J. Moon, J. S. Son, I. C. Song, W. K. Moon and T. Hyeon, J. Am. Chem.
Soc., 2008, 130, 15573–15580.
22 J. Choi, Y. Jun, S. Yeon, H. C. Kim, J. S. Shin and J. Cheon, J. Am.
Chem. Soc., 2006, 128, 15982–15983.
2
2
3 L. M. Bronstein, X. Huang, J. Retrum, A. Schmucker, M. Pink, B. D.
Stein and B. Dragnea, Chem. Mater., 2007, 19, 3624–3632.
4 S Sun and H. Zeng, J. Am. Chem. Soc., 2002, 124, 8204–8205.
5 H Si, H. Wang, H. Shen, C. Zhou, S. Li, S. Lou, W. Xu, Z. Du and L.
S. Li, CrystEngComm, 2009, 11, 1128–1132.
2
2
Acknowledgements
This work was supported by the research project of the National
Natural Science Foundation of China (20771035), Program for
New Century Excellent Talents in University of Chinese Ministry
of Education, Hi-Tech Research and Development Program
of China (863 plan, 2006AA03Z3592), and Innovation Scien-
tists and Technicians Troop Construction Projects of Henan
Province.
2
6 D. Wang, T Xie, Q Peng and Y Li, J. Am. Chem. Soc., 2008, 130,
4
016–4022.
27 S Sun, H. Zeng, D. B. Robinson, S. Raoux, P. M. Rice, S. X. Wang and
G. Li, J. Am. Chem. Soc., 2004, 126, 273–279.
8 C. Zhou, H. Shen, Y. Guo, L. Xu, J. Niu, Z. Zhang, Z. Du, J. Chen and
L. S. Li, J. Colloid Interface Sci., 2010, 344, 279–285.
9 X Jia, D Chen, X. Jiao and S. Zhai, Chem. Commun., 2009, 968–970.
30 X. Z. Lin, X. Teng and H. Yang, Langmuir, 2003, 19, 10081–10085.
2
2
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Dalton Trans., 2010, 39, 10984–10989 | 10989